Concretes and reinforced concretes with a low expansion coefficient

ABSTRACT

Concrete is known to have a significant heat expansion coefficient, of about 12.10 6 m/m/*C, which has to be considered in designing structures and which can also affect the cost of such structures considerably. The present invention relates in particular to concrete and self-prestressed reinforced concrete with a low expansion coefficient and density. The concrete and reinforced concrete products are suitable for all purposes involving highway construction, large slabs, etc., for the nuclear power industry. Such products can also be used in producing strong and light-weight units that can withstand heat.

AU 116 EX uuucu ounce 1 atcut 1 1 Magnan CONCRETES AND REINFORCEDCONCRETES WITH A LOW EXPANSION COEFFICIENT [75] Inventor: Regis Magnan,Montelimar, France [73] Assignee: S. A. Ciments LaFarge, Paris,

France [22] Filed: Jan. 4, 1971 [21] Appl. No.: 103,775

[30] Foreign Application Priority Data OTHER PUBLICATIONS Berry andMasen, Minerology," W. H. Freeman &

[451 Feb. 5, 1974 Go, pgs. 539-540 (1959). Lea and Desch, The Chemistryof Cement and Concrete, Edw. Arnold & Sons, pgs. 459-460, 483-487, 493,499-500 (1956).

Primary Examiner-James E. Poer [57] ABSTRACT Concrete is known to have asignificant heat expansion coefficient, of about 12.106 m/m/C, which hasto be considered in designing structures and which can also affect thecost of such structures considerably.

The present invention relates in particular to concrete andself-prestressed reinforced concrete with a low expansion coefficientand density.

The concrete and reinforced concrete products are suitable for allpurposes involving highway construction, large slabs, etc., for thenuclear power industry. Such products can also be used in producingstrong and light-weight units that can withstand heat.

12 Claims, 1 Drawing Figure CONCRETES AND REINFORCED CONCRETES WITH ALOW EXPANSION COEFFICIENT The present invention relates to a new processfor preparing concrete with a low expansion coefficient, and to aprocess for preparing self-prestressed reinforced concrete. Theinvention also relates to the concrates and reinforced concretesobtained by such a process.

Since the concrete products obtained by the present invention arecharacterized by a low expansion coefficient, such products areextremely useful in the building and public works areas.

Concrete is known to have a significant heat expansion coefficient undernormal conditions of use, of around 12-10 m/m/C, and this characteristicmust be considered in designing structures. It can also affect the costof such structures considerably, since it depends directly on thequality of the aggregates used in making the concrete, and in particularon their expansion coefficients. The aim has therefore been to findnatural or artifical solids with very low expansion coefficients.

Among natural minerals, particular attention was drawn to cordierite,with the formula 2MgO, 2Al,O;,, 5SiO which has an extremely lowexpansion coefficient, i.e., about 0.5310 F. Singer and Cohn (Ber.Deutsch. Ker. Ges. 10, p. 269 1929-), reported that cordierite can beproduced synthetically using a purely ceramic method by mixing a numberof minerals such as talc, plastic clay and alumina, in the proportionsrequired for the composition of cordierite. Other minerals, such asserpentine, magnesite and kaolin, can also be used. According to F.Singer and Cohn, the mixtures thus obtained are formed by pressing, thenbaked at l,4lC.

The present invention relates to a process for preparing concretes witha low expansion coefficient, in which cordierite or expanded cordieriteaggregates, which may br partly crushed, are mixed in such a way as topresent the desired grain-size range; a binder is then added, possiblyafter the adding of a proportion of fines.

According to one embodiment of the process involved in the presentinvention, these aggregates are obtained from an unbaked mixture formedfrom substances in the proper proportions required to obtain cordierite;these are crushed, possibly in the presence of an expansion agent, andthen formed into granules of suitable size and shape, which are thensintered at a temperature between 1,200 and l,450C, and preferably ataround 1,350 to 1,390C.

According to another embodiment of the process involved in the presentinvention, granules of suitable size and shape are obtained by any knownmethod, such as granulation or different extrusion processes.

According to yet another embodiment of the process involved in thepresent invention, the raw materials re quired to produce the cordieriteare, preferably, selected from substances such as kaolin, magnesite,alumina, bauxite, serpentine, talc and clay.

According to yet another embodiment of the process involved in thepresent invention, the expansion agent is added to the unbaked mixturein proportions ranging from 3 to percent in weight. 1

According to another embodiment of the process involved in the presentinvention. the expansion agent is a sulphate such as Ca, Mg, Na. Al, orFe sulphates, or

carbonate such as Li, Ca,Sr or Ba carbonates, or even an organicsubstance such as fuel-oil.

According to another embodiment of the process involved in the presentinvention, the expansion agent is anhydrous or hydrous calcium sulphate.

According to another embodiment of the process involved in the presentinvention, the aggregates may consist wholly or in part of cordierite,for instance 30 to 50 percent in weight of cordierite granules of morethan 5 mm in size; the proportion of fines may be sand, cordierite orany other material that can form an aggregate.

According to yet another embodiment of the process involved in thepresent invention, the binder may be any suitable binder such ashydraulic, chemical, ceramic or polymer-based binder.

The present invention also relates to the concrete thus obtained, with alow expansion coefficient, the aggregate content of which consistswholly or in part of cordierite, and relates in particular to concretewith adensity of less than 2.1.

The concrete involved in the present invention can be used to makeobjects that have to withstand thermal impacts and cycles, such as slabsand superstructures for tip-trucks in tunnel furnaces for the tile andbrick industry, heating covers, rotary cement-plant furnaces,blast-furnace mouths, saggers for the ceramic industry.

The present invention also relates to refractory substances obtainedfrom concrete consisting of cordierite and a chemical binder (phosphatesor sulphates), ceramic binder (stoneware clay with high magnesiumcontent), or polymer-based binder (aluminium hydroxychlorides orpolyphosphates').

The present invention also relates to a process for preparingself-prestressed concrete, in 'which reinforced concrete is hydrated ata fairly high temperature, of around C, for example and then cooledslowly.

According to another embodiment of the said process for preparingself-prestressed concrete, reinforced concrete is hydrated at atemperature roughly equivalent to room temperature, and then cooled bycontact with products such as liquefied gas, at the temperature of suchgas.

The present invention also relates to the reinforced concrete andself-pre-stressed reinforced concrete, and

in particular the tri-axial self-prestressed reinforced concreteobtained by means of the preparation process involved in the presentinvention.

The present invention also relates to any tank, and particularly anyliquefied gas-storage tanks, made from the said concrete,

Concretes and reinforced concretes according to the present inventionare particularly useful because of their low expansion coefficient andlow density, especially for civil engineering purposes (motorwayconstruction, large slabs without expansion joints) and the nuclearpower industry.

Other aims and advantages of the present invention will be apparent fromreading the examples below, which are non-restrictive, and whichrepresent various embodimentsof the invention.

EXAMPLE 1 Cordierite, with the formula 2MgO, ZAI O 5SiO is prepared fromkaolin, sintered magnesite and chinal y Sand, an nalysis Of which isgiven in table 1 below. tensile strength in Splitting test after 7 days25 bars TABLE 1 Kg per Materials SiO TiO A1 0,, Fe O CaO MgO H 1cordiused sprite Kaolin 50.9 0.45 34.5 0.36 l 0.25 10.8 1,010 Sinteredmagnesite 1.5 0.35 0.5 2.75 94.3 0.1 145 China-clay sand 79 119 Thismixture is first granulated in a plate-type tilted rotating granulator,to which approximately 15 percent 0 water is added. The unbakedgranules, the diameters of which range from 5 to 25 mm, are then driedand baked to between 1,350 and 1,370C, being held at this temperaturefor 15 minutes.

The only impurities revealed by an X-ray examination are tracesofmullite. The expansion coefficient between 0 and 90C, measured on asintered test-piece, is approximately 0.5-'.

For comparative testing purposes, cordierite aggregates were preparedfrom the necessary quantities of talc and serpentine, corrected withbauxite. The properties of aggregates obtained using impure naturalmaterials are slightly modified because of the impurities present, suchas FeO replacing MgO or Fe O How- 'ever, as long as the actual andtheoretical compositions of the cordierite are not too different, thequalities of the product remain acceptable.

EXAMPLE 2 Expanded cordierite can be obtained during baking, using thesame materials as above, but with the addition of3 to 5 percent SO Ca.Baking is done at 1,4100C. For expansion to take place normally, thetemperature must rise rapidly. This means that there is a considerabletemperature-lag between the core of the granule and its surface. The gasproduced in the granule core cannot escape becuase of the viscosity ofthe surface.

When expansion has occurred, the material has to be cooled quickly, toprevent the gas emitted from escaping from the granules. The temperatureshould therefore be held for only a short period at the end of baking.

1n the present example, the gas-generating element is SO Ca, whichdecomposes at around 1,000C into S0 which probably in turn breaks downinto SO and 0 The expanded substance obtained in cordierite, as is shownby the X-ray diffraction spectrum; the lime resulting from thedecomposition of the SO Ca forms part of the solid solution.

The specific weight is much less than 2, generally between 0.6 and 2.

This is a particularly valuable application, since the product can beused for lightweight objects that can withstand heat shocks.

EXAMPLE 3 Concrete is prepared from cordierite aggregates, in theproportions shown in table 2. obtained using the method involved in theinvention. The concrete has the following properties:

density when placed 2.106

compressive strength after 7 days on cubes of 14 18370 bars expansioncoefficient between 0 and C. 3.6'10' m/m/C.

A mixed concrete is prepared from cordierite aggregates with agrain-size of more than 5 mm obtained in Example 1 (the expansioncoefficient of the concrete depends mainly on the coefficient of thecoarsest aggregates from which it is produced), and a proportion offines, with a grain-size of less than 5 mm, consisting of Seine sand, inthe proportions shown in table 3 below.

TABLE 3 Grain-size Kg Cordierite 16 20 227 333 12.5- 16 10 12.5 236 8-10 142 5 -8 55 Seine sand 0 5 683 CPA 325 350 Water 146 lt.

This concrete has the following properties:

density when placed 2.172

Compressive strength after 7 days on cubes of 14 18420 bars tensilestrength in splitting test after 7 days 20 bars expansion coefficientbetween 0 .and 90C 185.9 10- m/m/C.

EXAMPLE 5 Refractory concrete is produced from cordierite granulesaccording to Example 1. mainly spherical, crushed, to produce a Fullercurve ofO 5 mm (FIG. 1), curves 0 0.2, 0.2 2, 2 5.

The composition in weight, with an aluminous cement basis, is asfollows:

Grain-size Kg Granulate 2 5 540 0.2 2 595 0.2 280 Aluminous cement 500Water 270 It.

The dilatometric curve shows very slight variations: 0.5 percentswelling at l,200C and 0.25 percent final shrinkage. This dimensionalstability is also confirmed 10 by measurement of post-variations.

DEFLECTION UNDER 2 BAR LOAD sum 0.5% 1% 2% 3% 4% 5% MECHANICAL STRENGTHSAT 20 7 days After 6 hrs at 800 After 6h at 1100 Bending CompressionBending Compression Bending Compression This application is particularlyuseful in the case of Pyroscopic strength of the concrete 1,380 tanksconsisting of an outer heat-insulating shell, an in- 113 221gi gbg' i 0termediate sealing shell, and an internal prestressed after 6 hrs at1100*; i 0 concrete shell (to increase overall solidity), which l 6 a!800: comes into contact with the cold 1i u'd d w loss of weight after 6hrs at 1.100: 10.9 q l an hlch' It will be noted that the mechanicalstrengths, and the start of deflection under load differ hardly at allfrom the corresponding properties of an aluminous cement fire-clayconcrete.

EXAMPLE 6 When the reinforcing bars in reinforced concrete have a muchhigher expansion coefficient than the surrounding concrete, a rise intemperature may cause the reinforced concrete to burst, by setting uptensile stresses that are above the acceptable limit.

To avoid this, such concrete is hydrated at a temperature of 80C andthen cooled slowly, so as to bring about self-prestressing of the wholeunit; this prestressing may also be tri-axial.

Stress levels of up to 140 bars may be involved. The steel is placedgradually under tension during cooling, not abruptly as in conventionalmethods.

EXAMPLE 7 Concrete corresponding to Example 3 is used to make tanks tohold liquefied gases at low temperature, the inner shells of which areof concrete.

This removes the need for hooping, and the problems where the amount ofliquid in the tank varies, may show temperature gradients vertically.

EXAMPLE 8 Refractory concrete is prepared from cordierite granulesaccording to Example 1, mainly spherical, crushed, and from aluminouscements.

40 The composition by weight is as follows:

Physical measurements produce the following results.

DEFLECTION UNDER 2 BAR LOAD 0.5 1 2 5 10 k 1nvo1ved in covering cablesin the hooped concrete 1270 I 330 I I 380 1390 I395 tanks normally used.

MECHANICAL STRENGTHS At 20C 7 days After 6 hrs at 800C After 6 hrs at 1C Bending Compression Bending Compression Bending Compression Pyroscopicstrength of the concrete l 46()C Dimensional post-variations at l,l()C0.18 Average expansion coefficient 3.10

Naturally, the invention is in no way confined to the examples givenabove: many variants will be possible for someone skilled in the art,depending on the applications involved and without implying anydeparture from the spirit of the invention. In particular, it is notnecessary for the product to correspond exactly to the stoichiometricrequirements laid down in the formula for cordierite, although it isclear that the more they differ from them, the more will the specificproperties be affected.

What is claimed is:

l. A process for preparing concrete having a low expansion coefficientwhich comprises mixing a composition consisting essentially ofcordierite granules with a binder selected from the group consisting ofportland, aluminous and white cements to obtain the desired product.

2. A process according to claim 1, wherein the cordierite granules aremore than about 5 mm in size.

3. A process according to claim 1, wherein the cordierite is granulatedin the presence of an expansion agent, and followed by sintering at atemperature between about l,200 and l,450C.

4. A process according to claim 1, wherein the cordierite is preparedfrom kaolin, magnesite, alumina, bauxite, serpentine, talc and clay.

5. A process according to claim 3, wherein the expansion agent is addedto the mixture in proportions ranging from about 3 to 5 percent inweight.

6. A process according to claim 5, wherein the expansion agent is asulphate selected from the group consisting of Ca, Mg, Na, and Fesulphates.

7. A process according to claim 5, wherein the expansion agent is acarbonate selected from the group consisting of Li, Ca, Sr and Bacarbonates.

8. A process according to claim 5, wherein the expansion agent is fueloil.

9. A process according to claim 6, wherein the expansion agent isanhydrous or hydrous calcium sulphate.

10. A process according to claim 1, wherein the granules consist whollyor 30 to 50 percent by weight of cordierite granules more than 5 mm insize and a proportion of fines of the group consisting of sand,cordierite. or any other material that can form an aggregate.

11. A concrete product obtained by the process of claim 1 having a lowexpansion coefficient and a density of less than 2.1 and the aggregatecontent consisting wholly or in part of cordierite.

12. A process according to claim 1. wherein the cordierite granules aremixed with sand and a binder selected from the group consisting ofPortland cement. aluminous cement. and white cement with a high aluminacontent, in the presence of water.

2. A process according to claim 1, wherein the cordierite granules aremore than about 5 mm in size.
 3. A process according to claim 1, whereinthe cordierite is granulated in the presence of an expansion agent, andfollowed by sintering at a temperature between about 1,200* and 1,450*C.4. A process according to claim 1, wherein the cordierite is preparedfrom kaolin, magnesite, alumina, bauxite, serpentine, talc and clay. 5.A process according to claim 3, wherein the expansion agent is added tothe mixture in proportions ranging from about 3 to 5 percent in weight.6. A process according to claim 5, wherein the expansion agent is asulphate selected from the group consisting of Ca, Mg, Na, and Fesulphates.
 7. A process according to claim 5, wherein the expansionagent is a carbonate selected from the group consisting of Li, Ca, Srand Ba carbonates.
 8. A process according to claim 5, wherein theexpansion agent is fuel oil.
 9. A process according to claim 6, whereinthe expansion agent is anhydrous or hydrous calcium sulphate.
 10. Aprocess according to claim 1, wherein the granules consist wholly or 30to 50 percent by weight of cordierite granules more than 5 mm in sizeand a proportion of fines of the group consisting of sand, cordierite,or any other material that can foRm an aggregate.
 11. A concrete productobtained by the process of claim 1 having a low expansion coefficientand a density of less than 2.1 and the aggregate content consistingwholly or in part of cordierite.
 12. A process according to claim 1,wherein the cordierite granules are mixed with sand and a binderselected from the group consisting of Portland cement, aluminous cement,and white cement with a high alumina content, in the presence of water.